Optical lithography has dominated the fabrication of integrated circuits for over 30 years. The general process involves back illuminating a mask with optical radiation, reducing the image of the mask with de-magnifying optics, and then imaging the pattern onto a substrate covered with a layer of photo-resist. Then, with the appropriate photo-resist, the substrate surface (covered with photo-resist) is chemically treated to remove those areas of the photo-resist, which were optically illuminated, thereby transferring a de-magnified image of the mask into the photo-resist. Subsequent chemical etching steps may be utilized to complete the process of transferring a de-magnified image of the mask onto the surface of the substrate material.
The constant competitive driving force in integrated circuits is for smaller and faster devices. Optical lithography has taken the state of the art down to dimensions where the diffraction of light has become the major limiting variable. Creating features smaller than the wavelength of the illuminating light has led to creative optical techniques such as off-axis illumination and phase-shifting masks. Even so, initially utilizing krypton fluoride excimer lasers at 248 nanometers (nm), and more recently argon fluoride excimer lasers operating at 193 nm, the industry standard today for narrow linewidths utilizing optical lithography hovers somewhere around 80 nanometers, slightly less that ½ the wavelength of the 193 nm illuminating laser. Also, at these short ultraviolet (UV) wavelengths material science issues become a practical limiting factor. For example, there are few materials that have sufficient UV transmission at these wavelengths to be used as either refractive lenses in the de-magnifying relay optics or as substrates for the mask assembly.
Given this, several non-optical lithography techniques have been explored by the semiconductor industry. Direct write electron-beam (E-beam) lithography has been researched and commercialized given its potential of wavelengths in the nanometer range. Commercial direct-write E-beam devices are readily available today with resolutions down to 50 nm and slightly below, however, the direct write devices are slow to process a large scale wafer and the search continues for a faster way to utilize the potential resolution available from electron-beam lithography.
Planar electron beam lithography has been investigated for over 20 years with limited commercial success. One configuration commonly referred to as an M-I-M (Metal-Insulator-Metal) planar electron beam lithography (PEBL) device has been constructed and has demonstrated partial technical success. The M-I-M devices consist of an insulator material sandwiched between two conducting metal materials. The individual metal and insulator layers may be made sufficiently thin that when an electrical voltage is applied across the device, electrons from the cathode (the metal with the negative electrical potential) may be driven by the electro-static forces to quantum tunnel into and through the first part of the insulating layer, then drift through the remainder of the insulating layer and anode metal (at the positive potential) and exit the device as free particles, essentially an electron gun. Devices of this type have been fabricated in cross-sectional dimensions as large as 1 inch square, and with appropriate sub-micron masking of the output electron beam, this device configuration opens the possibility of projecting patterns at a 1:1 (one-to-one) ratio in the resist-covered substrate. Also, it has been demonstrated that the exposure time to transfer the entire pattern to the photo-resist in this configuration can be as small as {fraction (1/10)} of a second, which may allow for rapid processing of large commercial wafers utilizing a step-and-repeat procedure. This rapid pattern transfer rate may give the emerging planar electron beam emitter a technical/commercial advantage over the traditional electron beam devices that write the pattern sequentially in the resist similar to how a television raster scans the screen with a small pencil beam.
However, the useful life of the planar electron emitting devices have historically been sufficiently short so as to limit their applicability to commercially viable manufacturing processes. In view of this, there is a need for a method or technique to prolong the lifetime of planar electron beam emitters for application to electron beam lithography.